: The endothelium serves as a critical component in the maintenance of vascular homeostasis. Alterations in endothelial cell production of oxidants contribute to defective vascular function and are implicated in the pathogenesis of diverse vascular diseases. However, the tissue sources of enhanced vascular cell oxidant production and the sites of oxidant action have not been determined with certainty. Recently, recognition of a) elevated plasma levels of circulating xanthine oxidase activity in diverse disease processes (sepsis, hypercholesterolemia, post- liver transplantation) and b) the ability of xanthine oxidase to specifically bind to endothelium with resultant increases in intracellular xanthine oxidase activity provide potential mechanism(s) by which non-endothelial cell-derived xanthine oxidase could contribute to endothelial oxidant production. While enhanced production of superoxide can diminish nitric oxide bioavailability by virtue of its reaction with nitric oxide and thus, concomitantly yield secondary oxidants, the potential for superoxide and other cellular oxidants to directly modify the activity of endothelial nitric oxide synthase has not been fully explored. From this foundation of understanding, it is hypothesized that increases in cell-associated xanthine oxidase modulate endothelial-dependent vascular function. To address this hypothesis, the following Specific Aims will be pursued: 1) Characterize vascular cell interactions with circulating xanthine oxidase. The kinetics of cell binding and uptake of circulating xanthine oxidase/dehydrogenase will be determined. In addition, xanthine oxidase circulating and ultimate tissue distribution will be defined in vivo. 2) Explore the impact of elevated xanthine oxidase-derived products (reactive species, uric acid) on expression and activity of endothelial nitric oxide synthase. The effects of xanthine oxidase on transcription and translation of endothelial nitric oxide synthase will be ascertained as well. The vascular functional consequences of increased xanthine oxidase activity will also be determined. Upon successful completion of the proposed aims, a) the contribution of xanthine oxidase to endothelial cell oxidant production will be better defined, b) detailed mechanistic information will be available regarding the presence, reactions and regulation of specific oxidative pathways that modulate endothelial nitric oxide synthase expression and activity and c) new insight will be gained for prospectively devising mechanism-directed pharmacologic strategies.